linux-kernel - [PATCH v2 2/3] lockref: use arch_mutex_cpu_relax() in CMPXCHG

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Date:	Sat, 28 Sep 2013 12:58:10 +0200
From:	Heiko Carstens <heiko.carstens@...ibm.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Tony Luck <tony.luck@...el.com>, Waiman Long <waiman.long@...com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Heiko Carstens <heiko.carstens@...ibm.com>
Subject: [PATCH v2 2/3] lockref: use arch_mutex_cpu_relax() in CMPXCHG_LOOP()

Make use of arch_mutex_cpu_relax() so architectures can override the
default cpu_relax() semantics.
This is especially useful for s390, where cpu_relax() means that we
yield() the current (virtual) cpu and therefore is very expensive,
and would contradict the whole purpose of the lockless cmpxchg loop.

Signed-off-by: Heiko Carstens <heiko.carstens@...ibm.com>
---
 lib/lockref.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/lib/lockref.c b/lib/lockref.c
index e294ae4..6f9d434 100644
--- a/lib/lockref.c
+++ b/lib/lockref.c
@@ -12,6 +12,14 @@
 #endif
 
 /*
+ * Allow architectures to override the default cpu_relax() within CMPXCHG_LOOP.
+ * This is useful for architectures with an expensive cpu_relax().
+ */
+#ifndef arch_mutex_cpu_relax
+# define arch_mutex_cpu_relax() cpu_relax()
+#endif
+
+/*
  * Note that the "cmpxchg()" reloads the "old" value for the
  * failure case.
  */
@@ -28,7 +36,7 @@
 		if (likely(old.lock_count == prev.lock_count)) {		\
 			SUCCESS;						\
 		}								\
-		cpu_relax();							\
+		arch_mutex_cpu_relax();						\
 	}									\
 } while (0)
 
-- 
1.8.3.4

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